There presently exist brake pressure control devices which are operably interposed between vehicle air springs and a brake pressure supply valve so as to be responsive to the pressure of fluid prevailing in the air springs, which reflects the degree of vehicle load, for controlling the pressure of braking fluid supplied to the brake cylinders in accordance with the prevailing vehicle load and thereby effect ideal braking force for the given vehicle load. The usual construction of presently known brake pressure control devices includes first diaphragm means arranged in oppositely facing relation to second diaphragm means with a precalibrated spring compressed therebetween and acting on the respective facing sides of the two diaphragm means. The respective sides of the two diaphragm means are subjected, respectively, to prevailing air spring pressure and to pressure delivered to the braking apparatus, so that the force differential between the air spring pressure and the precalibrated spring across the first diaphragm means causes the second diaphragm means to effect operation of a self-lapping valve member to effect delivery and maintain pressure to the braking apparatus at a ratio determined by prevailing air spring pressure and the calibrated spring.
In such a conventional device, as immediately above described, the effective area ratio between the first and second diaphragm means, and the calibration of the spring disposed therebetween, are both fixed and cannot be adjusted. Thus, when used on different vehicles characterized by different speed retardation requirements, different air springs, and different vehicle loads, etc., the control devices must be replaced by other devices having diaphragms and springs that will meet the requirements for the particular vehicles, if it is desired to obtain ideal braking action. This would necessitate having many different brake pressure control devices which would be costly for manufacturing them.